Lithium ion cells are the fastest-developing storage cells at present, and are broadly used in portable electronic products and household appliances. With the increasing petroleum crisis and global warming problems, it is an urgent need to develop green material and energy-related applications. Among these applications, the development of the electric vehicle is important to decrease petroleum usage in the face of global warming. The lithium cell is the most promising cell that can be used in an electric vehicle, wherein the lithium salt used for portable electronic products is LiPF6. LiPF6 has poor thermal stability, and it tends to decompose into LiF and PF5 when the operating temperature is higher than 60° C. (J. Power Sources 81 (1999) 119; J. Power Sources 89 (2000) 206), wherein PF5 and LiPF6 react with water to produce corrosive HF in the presence of trace water and will cause the corrosion of the current collector layer, deterioration of the solid electrolyte interface (SEI) and damage of the electrode material. Currently, lithium cells using LiPF6 cannot be directly applied in electric vehicles because the cells need to be modified so as to repeatedly recharge under a high temperature (>60° C.) for a long time. The most important part of the lithium cell that can be used in electric vehicles is a thermally stable lithium salt, which is the object of the present invention.
The lithium cell primarily consists of graphitic carbon as an anode, lithium transition metal oxide as a cathode and an electrolyte solution having high conductivity. The electrolyte of the lithium ion is formed by dissolving the lithium salt in a non-aqueous solvent, wherein the primary solvents are alkyl carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and propylene carbonate (PC).
From the 1990s to the present, LiPF6 has been the dominant commercial lithium salt. LiPF6 has very high conductivity and high solubility in alkyl carbonates, it is electrochemically stable under the operating voltage (<4.2V vs. Li) and can be used at a temperature between −40 to 50, therefore, there is almost no other commercial lithium salt that can replace LiPF6. However, LiPF6 lithium salt cannot be used in a hot environment (>60° C.) because the following reactions will occur when the temperature is higher than 60° C.:LiPF6LiF+PF5 wherein LiPF6 and PF5 react with water to produce corrosive HF in the presence of trace water and result in a decrease in the performance of the cell:LiPF6+H2O→POF3+LiF+2HFPF5+H2O→POF3+HFTherefore, because LiPF6 lithium salt cannot be used in a hot environment (>60° C.), there is an urgent need to develop a lithium salt that can be used at high temperatures and is suitable for electric vehicles.
Any lithium salt that can be applied in lithium ion cells must have the following properties: high conductivity, good thermal stability, nontoxicity, safety and electrochemical stability under the potential of a fully charged cell (4.1V vs. Li), but few lithium salts have all these properties. Lithium salts can be classified as inorganic lithium salts and organic lithium salts. Examples of inorganic lithium salts are LiClO4, LiAsF6, LiPF6, LiBF4, Li2B12F12 (referring to J. Power Sources 193 (2009) 851), wherein LiClO4 may explode when it is used, LiAsF6 is highly toxic, LiBF4 has lower ion conductivity and Li2B12F12 also has low ion conductivity due to large anion. These significant disadvantages prevent these lithium salts from being commercialized. Examples of organic lithium salts are LiN(SO2CF3)2(LiTFSI), LiN(SO2C2F5)2(LiBETI), LiC(SO2CF3)3, LiCF3SO3(LiTF), LiC(SO2CF3)2 (referring to European Polymer Journal. 43 (2007) 5121). However, these lithium salts cannot be commercialized due to the disadvantage of causing the aluminum collector layer to corrode. Other organic lithium salts such as Lithium tetrakis(haloacyloxy)borates (Li[B(OCOBX)4]), Lithium bis(oxalate)borate (LiBOB), LiPF3(CF2CF3)3 (LiFAP) (referring to European Polymer Journal. 43 (2007) 5121) cannot be commercialized due to their low ion conductivities.
In the development of high temperature lithium salts, the lithium salt containing heterocyclic anion is main strategy, (referring to Electrochim. Acta 55 (2010) 1450) such as lithium 4,5-dicyanotriazole (LiDCTA). LiDCTA was successfully used in a Polyethylene oxide (PEO) system, but it was not ideal for an EC/DMC system.
One interesting salt is an organic imidazolide lithium salt, lithium bis(trifluoroborane)imidazolide (Lilm(BF3)2), and this lithium salt has been used in forming lithium ion cells (see J. Am. Chem. Soc., 122, 9560 (2000), J. Electrochem. Soc., 149, A355 (2002)). It has been demonstrated that a cell with an electrolyte containing Lilm(BF3)2 has comparable performance to a cell containing LiPF6. Unfortunately, a strong base n-butyllithium (n-BuLi) was required to synthesize this type of Lilm(BF3)2 lithium salt, and a lithium salt made in this way contains impurities that are difficult to remove. As a result, such a lithium salt shows poor over-change safety and poor performance under high temperature. The anion of lithium salt with good thermal stability is important for the safety of the cells, the thermal stable lithium salt would prevent the cell from smoking and firing even the cell is over-charged. It would therefore be highly desirable to develop a lithium salt with good conductivity, good thermal stability and good electrochemical safety during the charge and re-charge operations of the cells, in order to enhance the safety of the lithium ion cell.
The present invention discloses an organic metal salt having good solubility (>1.0 M) in alkyl carbonates solvents that are commonly used in lithium cells. When dissolving such a salt in alkyl carbonates to prepare a non-aqueous electrolyte, this non-aqueous electrolyte has good conductivity (6.85 mS/cm). The lithium salt of the present invention had excellent thermal stability (i.e. the initial decomposition temperature>200° C.) and oxidation resistant electrochemical stability (i.e. anodic limit>5.0V vs. Li). The present invention also discloses a simple method suitable for preparing a cyano benzimidazole metal salt.